The invention relates to the field of watermarking an information signal and detecting a watermark.
Watermarks are perceptually invisible messages embedded in information signals such as multimedia material, e.g. audio, still pictures, animations or video. Watermarks can be used to identify the copyright ownership of information. They allow a copyright owner to trace illegal copies of his material by inspecting whether his watermark is present in said copies.
Watermarks are embedded in an information signal by modifying data samples of the signal (e.g. audio samples of an audio signal, pixels of an image, transform coefficients of a transform-coded signal, etc.) such that the original is not perceptibly affected. Various methods of watermarking are known in the art. For example, pixels of an original image are slightly incremented or decremented in accordance with corresponding bits of a binary watermark pattern.
In order to detect whether an information signal has an embedded watermark, the signal is subjected to a statistical analysis. The statistical analysis yields a parameter, hereinafter referred to as xe2x80x9cdecision variablexe2x80x9d, which indicates to which extent the watermark is present in the signal. For example, if an image signal is watermarked by incrementing or decrementing its pixels in accordance with a watermark pattern, the decision variable may be the amount of correlation between the signal and an applied reference copy of the watermark. If an image is watermarked by modifying selected pixels, a prediction for said pixels is calculated from temporally or spatially adjacent pixels. The decision variable may then be the number of pixels being sufficiently different from their prediction.
Watermark detectors generate a binary output signal indicating xe2x80x9cwatermark foundxe2x80x9d or xe2x80x9cno watermark foundxe2x80x9d. That is achieved by comparing the decision variable with a predetermined threshold. If the value of the decision variable exceeds the threshold, the watermark is considered to be present in the signal.
It is an object of the invention to provide a method and arrangement for detecting the presence of a multi-bit watermark value in the signal. It is also an object of the invention to provide a method and arrangement for accommodating such a multi-bit watermark value in images of a video signal.
This is achieved in by a method of detecting a multi-bit watermark in a signal, each bit of said multi-bit watermark being represented by a respective watermark subpattern, comprising the steps of: determining, for each watermark subpattern, a correlation value indicating the extent to which said subpattern is present in the signal; combining said correlation values to detect whether the watermark is present in the signal irrespective of its multi-bit value; and comparing said correlation values and the corresponding bits of an applied multi-bit value to obtain an indication of to which extent the detected watermark has said applied multi-bit value.
The invention is based on the desire to embed a multi-bit watermark in a signal. This is achieved by representing each bit of the watermark by a respective watermark subpattern. A straightforward method of detecting the watermark would include using a conventional watermark detector for each individual bit of the watermark by detecting the presence of the corresponding subpattern. However, as already mentioned above, detecting a watermark is a statistical analysis process which may result in false decisions. The reliability of each individual detection is weak because its signal strength is 1/nth (n is the number of bits) of the energy in the full watermark.
The inventors have recognized that is advantageous to detect the presence of the watermark irrespective of its multi-bit value at maximum reliability, and compare the (Hamming) distance between the intermediate results (i.e. the extents to which individual subpatterns are present in the signal) and the multi-bit value which is to be checked. The reliability of the multi-bit detection is now comparable to the reliability of the detection of an entire, full strength watermark.
The invention further provides a method of embedding a multi-bit watermark in a video image by adding a watermark pattern to said image, comprising the steps of: dividing the video image and said watermark pattern into blocks, the blocks of the watermark pattern constituting subpatterns; and representing each bit of the multi-bit value by adding or not adding a subpattern to the corresponding image block.